420Originating from a bunch of San Rafael High School' teenagers who used to get high around 04.20 PM, the term 420 is used to refer to anything cannabis related. It's a subculture code for marijuana culture. 

The actual 4/20 day as a celebration and protest started in Vancouver in 1995 by Canadian cannabis activist “Prince of Pot” Marc Emery.

Did you know that the number “420”, as a slang term, was popularized as code for smoking pot among the Greatful Dead fans?

Written in dateform, 4/20 being April 20, it's a day to celebrate cannabis friendly life worldwide.